This invention relates to improved monolithic ceramic capacitors, and to improved ceramic dielectric compositions for making such capacitors. Even more particularly, this invention relates to an improved ternary ceramic composition which exhibits a high dielectric constant, low sintering temperature, and useful dielectric constant temperature characteristics, and which is particularly suited for use in making monolithic ceramic capacitors having a Z5U rating.
As is well known by those skilled in the art, ceramic dielectric compositions consisting essentially of BaTiO.sub.3 modified with stannates, zirconates and titanates of alkaline earth metals are high dielectric constant materials that have been widely employed in the manufacture of monolithic capacitors. However, these compositions must be sintered at relatively high temperatures, e.g., in the range of 1300.degree.-1400.degree. C. These conditions require the use of electrode materials with high melting points, good oxidation resistance at elevated temperatures, minimal tendency to react with the ceramic dielectric during the sintering process, and sinterability at the maturation temperature of the dielectric. Due to these requirements the choices for electroding materials have been restricted to precious metals, such as for example platinum, palladium or combinations thereof.
Significant savings in electroding costs for capacitors of the type described could be realized if dielectric materials were sintered at temperatures below 1000.degree. C., and preferably below 960.degree. C. This would enable the use of less expensive metals such as silver, oxygen-retarding nickel alloys, and the like. Recent advancements in the art have suggested the use of BaTiO.sub.3 based ceramic dielectrics which can be sintered in the 1000.degree. C. area by incorporating a low melting liquid phase. These systems, however, often result in compositions having low dielectric constants (e.g. less than 4000) due to the low dielectric constant grain boundary phases formed from the low melting component. As a result, an increased number of metal electrode layers may be necessary to achieve a given capacitance value in a monolithic capacitor. It can easily be seen, therefore, that it would be most advantageous to retain dielectric properties, such as a high dielectric constant, in order fully to realize savings in electroding costs.
U.S. Pat. No. 4,078,938 has suggested using lead-ferrous-tungstate and lead-ferrous-niobate compounds for producing ceramic compositions that have low sintering temperatures and high dielectric constants, but the disadvantage of such compositions is that they cannot be used to produce satisfactory monolithic capacitors that also meet the Z5U rating of the U.S. Electronics Industries Association (EIA) Standard--i.e., a temperature coefficient range of +22% to -56% at an operating temperature range of +10.degree. C. to +85.degree. C. and an insulation resistance of 7.5.times.10.sup.9 ohms or higher.
U.S. Pat. No. 4,236,928, however, has disclosed a ternary ceramic system consisting essentially of Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3, Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3, and Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 and which can be used to produce capacitors which meet the Z5U rating of the EIA. This is made possible essentially by controlling the addition of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3. It is this latter component or additive which is responsible for modifying the curie temperature and temperature dependence of capacitance of the composition to meet various EIA standards. In general, an increase in Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 results in an increase in the curie temperature and a decrease in temperature dependence of capacitance. The disadvantage of this system, however, is that the addition of zinc in the form of the Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 not only tends to move the curie peak upwardly on the temperature scale of the associated TC curve, it also does little to flatten the curve.
One object of this invention, therefore, is to provide an improved ternary composition containing a compound which tends to flatten and to lower the curie peak for a ceramic composition of the type described.
It is an object also of this invention to provide an improved ternary ceramic composition which allows low temperature sintering, has a high dielectric constant, a low dielectric loss, high specific resistivity, and a temperature dependence of dielectric constant which meets Z5U rating of the EIA standard.
Another object of the present invention is to provide low temperature sintering compositions which meet EIA standards for temperature coefficient of capacitance which allow even smaller changes in capacitance vs. temperature, such as Y5R, X5S, and Z5R characteristics.
Still another object of the present invention is to provide a ceramic capacitor composition maneuvered to meet EIA standard dielectric constant vs. temperature characteristics such as Z5U, Y5R, X5S and Z5R by use of a Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 component, in addition to Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 and Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3.
Another object of the present invention is to incorporate 0.1 to 0.75 wt.% of a Mn(NO.sub.3).sub.2 solution in the above mentioned ceramic composition to improve dielectric losses at elevated temperatures and to improve insulation resistance.
It is also an object of the present invention to provide improved, multilayer ceramic capacitors which utilize the low temperature sintering dielectric composition and lower cost, high-silver-content buried electrodes.